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Henry Kolesnik wrote:
I know that any power not dissipated by an antenna is reflected back to the transmitter. Then the transmitter "reflects" this reflection back to antenna, ad nauseum until its all gone. I also know that a short or an open is required to reflect power and I'm searching for which it is, an open or a short. I'm inclined to think it's a virtual open but I'm at a loss to understand that and I wonder if someone has a good explanation or analogy and some math wouldn't hurt. Hank, EM reflections are covered on this web page. http://www.mellesgriot.com/products/optics/oc_2_1.htm In particular: "Clearly, if the wavelength of the incident light and the thickness of the film are such that a phase difference exists between reflections of p, then reflected wavefronts interfere destructively, and overall reflected intensity is a minimum. If the two reflections are of equal amplitude, then this amplitude (and hence intensity) minimum will be zero." "In the absence of absorption or scatter, the principle of conservation of energy indicates all "lost" reflected intensity will appear as enhanced intensity in the transmitted beam. The sum of the reflected and transmitted beam intensities is always equal to the incident intensity. This important fact has been confirmed experimentally." In order for (rearward-traveling) "reflected intensity" to "appear as (forward-traveling) enhanced intensity in the transmitted beam", the momentum of that (rearward-traveling) intensity must change directions. Thus, it appears that total destructive interference between two rearward- traveling reflected waves in a transmission line will reverse the direction of momentum of the energy in those canceled reflected waves. We need to change a few of your statements: Any power not dissipated or radiated by an antenna is reflected back. "Dissipation" means EM energy transformed into heat, according to the IEEE Dictionary. The transmitter/tuner end will not re-reflect 100% of the reflected energy unless there exists a short, open, pure reactance, or "total destructive interference" as explained in _Optics_, by Hecht. Besides a short or an open, a purely reactive impedance will cause 100% energy reflection. Apparently, so will "total destructive interference". Quoting from _Microwave_Transmission_, by J. C. Slater: "The method of eliminating reflections is based on the interference between waves. ... The fundamental principle behind the elimination of reflections is then to have each reflected wave canceled by another wave of equal amplitude and opposite phase." That's pretty clear. We get one set of rearward-traveling reflections at the match point. We get another set of rearward-traveling reflections at the antenna. If these two sets of reflections are equal in magnitude and opposite in phase at the match point, they cancel each other and the rearward-traveling momentum energy in those two waves is conserved by changing direction to become part of a forward-traveling wave. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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